Just a simple snippet to show how an aspx can be exported to Excel
Response.ClearContent();
Response.Buffer = true;
Response.ContentType = "application/vnd.ms-excel";
Response.AddHeader("content-disposition", "attachment; filename=" + "teste" + ".xls;charset=iso-8859-1");
StreamWriter sw = new StreamWriter(Response.OutputStream, System.Text.Encoding.GetEncoding("iso-8859-1"));
HtmlTextWriter htmlTextWriter = new HtmlTextWriter(sw);
this.RenderControl(htmlTextWriter);
Response.End();